The present invention generally relates to liquid dispensing technology and, more specifically, to adhesive dispensers using heated or unheated manifolds and valve modules to selectively dispense liquid adhesive.
Existing hot melt adhesive dispensers operate at relatively high temperatures, such as above about 250xc2x0 F. Present dispenser configurations have high temperature surfaces exposed to personnel. Considerable measures are taken to guard or insulate the dispensing equipment from nearby personnel. However, this also reduces the ease with which the equipment may be serviced by such personnel.
Many hot melt dispensers include a heated manifold for supplying hot liquid adhesive to one or more valve modules. Very often, these manifolds are heated by cartridge heaters or other heating elements contained within the manifold. The manifold may therefore contain high tolerance bores for receiving the heaters. Air gaps can exist between the heaters and the manifold resulting in localized hot spots or overheating. Over time, these hot spots will cause heater failure. In some cases, it may also be difficult to obtain highly uniform heating of a manifold through the use of internal heaters. For example, small manifolds or irregularly-shaped manifolds may not easily permit the use of cartridge heaters or cast-in-place heaters.
Present methods of supplying liquid hot melt adhesive can also result in adhesive stagnation and air pocketing. This contributes to char formation and related overheating problems which then adversely affect dispenser performance. Also, the typical circular cross sectional flow area of liquid supply passages is an inefficient heat transfer configuration. Many manifolds are also constructed of cast metal thus leading to lower strength threads and difficulty in accommodating a liquid filter.
Another problem arising when dispensing viscous liquids, such as hot melt or room temperature adhesive, relates to the formation of tailing, stringing or drooling of adhesive upon liquid cut-off. The inertial effects of fluid flow may prolong adhesive cut-off, therefore resulting in these undesirable effects. In a traditional valve arrangement, liquid adhesive flows parallel to a valve stem into the valve seat area. When the end of the valve stem is lifted from the seat, the flow path is relatively straight. As the valve stem approaches the seat, the liquid inertia combines with the decreasing flow area between the valve stem and the seat edge thereby resulting in increased liquid flow velocities. These increased velocities can lead to stringing, tailing or drooling of adhesive after cut-off. When dispensing hot melt adhesives, the same cut-off problems can arise if the adhesive is not maintained at the proper set point temperature in the nozzle.
It would therefore be desirable to provide dispensing apparatus for dispensing liquid hot melt or room temperature adhesive and overcoming problems in the art such as those mentioned above.
In one general aspect, the invention provides apparatus for dispensing liquid hot melt adhesive, including a manifold, a dispensing module connected with the manifold, a heater thermally coupled with the manifold and a thermally insulating cover structure surrounding the module and the manifold for preventing exposure of personnel to the hot manifold and module surfaces. The cover structure is preferably formed of a plastic material having a low thermal conductivity and preferably includes a plurality of outwardly projecting fins for further dissipating heat. Ideally, the outer edges of the fins are maintained at a temperature below a burn threshold temperature. Also in accordance with the invention, air spaces or gaps are formed between the cover structure and the module and between the cover structure and the manifold for decreasing heat transfer to the cover structure.
According to another feature of the invention, a thin film heater is bonded directly to the manifold. The thin film heater supplies heat directly through outer surfaces of the manifold. In this way, the manifold may be small and/or irregularly-shaped and still be heated in a uniform and efficient manner. Power consumption is also reduced, especially when combined with the thermally insulating cover structure. Preferably, the heater incorporates a sensor for temperature control purposes and may also incorporate a thermal fuse or thermostat for protection against overheating.
In one alternative, a manifold assembly comprises a manifold body including an inlet bore having an interior wall and a liquid supply passage communicating with the inlet bore. A heater is thermally coupled with the manifold body. A supply connector extends within the inlet bore and is configured therewith to provide better heat transfer and manufacturing advantages, such as thread elimination and alternative connection orientations. The supply connector includes an interior flow passage, an exterior annular recess disposed adjacent the interior wall of the inlet bore, and at least one port communicating between the interior flow passage and the exterior annular recess. The annular recess communicates with the liquid supply passage of the manifold. The inlet bore preferably extends completely through the manifold and is preferably a smooth bore. A pair of seals extend around the connector each respectively engaging the interior wall on opposite sides of the liquid supply passage. In one alternative, the connector further comprises a filter retained in the interior flow passage for filtering the liquid hot melt adhesive flowing into the exterior annular recess.
In another aspect of the invention, a valve is provided for dispensing viscous liquids, such as hot melt adhesives or room temperature adhesives. The valve includes a valve seat having an orifice and a sealing surface located around the orifice. A valve stem is movable between open and closed positions with respect to the valve seat and includes one end with a recess and a sealing edge located around the recess. The sealing edge is engaged with the sealing surface of the valve seat in the closed position and is spaced from the sealing surface in the open position. The recess is designed to provide a more tortuous flow path for the liquid to reduce the localized liquid flow velocities and thereby reduce undesirable cut-off effects, such as stringing, tailing or drooling of adhesive.
Another feature of the invention relates to a unique, temperature controlled valve module. More specifically, the valve module dispenses heated liquids at a predetermined set point temperature, such as in the case of the application temperature of a hot melt adhesive. The valve module includes a module body having a liquid cavity communicating with a dispensing orifice, a valve seat disposed generally between the liquid cavity and the dispensing orifice and a valve stem mounted for movement within the cavity between engaged and disengaged positions relative to the valve seat for selectively dispensing liquid from the dispensing orifice. In accordance with this aspect of the invention, a heating element is thermally coupled with the module body and a temperature sensor is also thermally coupled with the module body for detecting the temperature of the liquid. This coupling may be a direct incorporation within the module body or, for example, may be separate pieces in thermal contact. Advantageously, this configuration more accurately controls the liquid temperature at the desired set point temperature within the dispensing orifice or nozzle. This results in better cut-off and less stringing of viscous liquids, such as hot melt adhesive.
These and other advantages, objects and features of the invention will become more readily apparent to those of ordinary skill in the art upon review of the following detailed description of the preferred embodiment taken in conjunction with the accompanying drawings.